Computational insights into non-C2-symmetric BINOLate titanium catalyzed cyanation of aldehydes.

The mechanism and enantiochemistry of non-C2-symmetric BINOLate titanium (1 : 1) catalyzed cyanation of aldehydes have been investigated using density functional theory (DFT). Calculation results suggest that the mononuclear titanium–BINOLate complex might be the dominant catalyst structure of this cyanation reaction. Using benzaldehyde as the substrate, the cyanation reaction could be completed via the bifunctional LALB mechanism or the Ti-HCN mechanism. For the bifunctional LALB mechanism, HCN needs to be isomerized to HNC under the catalysis of the imine N atom of imidazole firstly, then it attacks the carbonyl C atom to form the C–C bond, followed by a hydrogen transfer step to yield product cyanohydrins. For the Ti-HCN mechanism, following the formation of catalyst–HCN complexes is a decomposition step of HCN, and then the decomposed HCN directs the aldehyde to generate the final product via a concerted transition state. Computational results demonstrate that both isomerization and decomposition of HCN are predicted as the rate-determining steps of two reaction mechanisms. The enantioselectivity of the title reaction is mainly controlled by the energetic performance of the C–C bond-forming transition states, which is influenced greatly by the solvent effects. The calculation results show that the (S)-configuration product would be the predominant product in CH2Cl2, which is consistent with the experimental results. [ABSTRACT FROM AUTHOR]